After a wellbore is drilled, an underreamer is oftentimes used to enlarge the diameter of the wellbore. Conventional underreamers have a body with a mandrel extending axially therethrough. The mandrel has an axial bore through which fluid flows. One or more cutter blocks are movably coupled to the body and adapted to transition from a retracted state to an expanded state.
The underreamer is run into the wellbore in the retracted state. In the retracted state, the cutter blocks are folded into the body of the underreamer such that the cutter blocks are positioned radially-inward from the surrounding casing or wellbore wall. Once the underreamer reaches the desired depth in the wellbore, the underreamer is actuated into the expanded state. In the expanded state, the cutter blocks move radially-outward and into contact with the wellbore wall. The cutter blocks are then used to cut or grind the wall of the wellbore to increase the diameter thereof.
The (radial) height of the cutter blocks is less than or equal to the (radial) distance between the outer surface of the mandrel and the outer surface of the body. As the height of the cutter blocks increases, so may the amount by which the cutter blocks are adapted to increase the diameter of the wellbore when in the expanded state. Conventional cutter blocks are adapted to increase the diameter of the wellbore between about 15% and about 25% from the original (i.e., pilot hole) diameter. When a larger increase in the wellbore diameter is desired, the first underreamer is pulled out of the wellbore, and a second, larger underreamer is run into the wellbore to further increase the diameter of the wellbore. Running multiple underreamers into the wellbore is a time-consuming process, which can lead to lost profits in the field.